1. Field of the Invention
The present invention relates to a control apparatus capable of improving a conductive state in a signal line and a signal line connection unit in an image forming apparatus such as a copying machine or a printer which uses an electrophotographic process.
2. Description of the Related Art
A thermistor is frequently used in detecting a temperature of a fixing device of the image forming apparatus. To detect the temperature of the fixing device, the thermistor is installed within the fixing device, and configured to transmit a detected signal to a control unit such as a central processing unit (CPU) via a signal line and a connector.
The control unit controls the fixing device to maintain a predetermined temperature based on the detected signal from the thermistor.
The thermistor applied to the fixing device is configured to have high detection accuracy in a high temperature, since the thermistor is installed in order to detect a temperature in heating and fixing toner on a transfer material, in other words, a temperature in a high-temperature area. A resistance value of the thermistor is usually about several hundreds to several kilo Ω. Further, the detection accuracy in a low-temperature area such as a room temperature is low, and a resistance value in the low-temperature area is usually several hundreds to several mega Ω.
Due to such characteristics, when a temperature of the fixing device is low, only a small amount of current (μA order) flows in the signal line and the connector to which the thermistor is connected because of a high resistance value of the thermistor. In such a case, the following problem occurs.
Since only a small amount of current flows in the signal line to which the connector is connected, when external noises enter the signal line, a detected signal of the thermistor on the signal line is easily disturbed. When the detected signal of the thermistor is disturbed, a temperature of the fixing device is falsely recognized.
When connectors are left unfitted or unconnected from each other, an oxide film may be formed on a surface of a pin of the connector. Once such an oxide film is formed on the pin of the connector, even if the connectors are fitted together to turn on power, the oxide film of the pin cannot be destroyed due to a low value of a flowing current, and an electric connection cannot be established.
When no electric connection is established in the thermistor, or noises are superimposed on a transmission path of a signal from the thermistor, a temperature may be detected higher or lower than an actual temperature.
When a high temperature is detected by mistake, the fixing device is controlled at a temperature lower than an original target temperature, and it may cause deterioration in image quality such as an occurrence of a fixing failure of a toner image on the transfer material. When a low temperature is detected by mistake, the fixing device is controlled at a temperature higher than the original target temperature, an it may cause a problem such as acceleration of deterioration of the fixing device.
To prevent contact failure of a connector caused by the oxide film, the following measures may be taken:
1. Plating the connector pin with a material which is difficult to oxidize (e.g., gold);
2. Forming the connector into a shape/structure that causes when the connectors are fitted together, the connector pins to rub each other to damage a surface of the opposite side, so that the oxide film is destroyed (e.g., Japanese Patent Application No. 8-50941); and3. Eliminating a contact failure by supplying a current which can destroy the oxide film in a contact point after the connectors are fitted together (e.g., Japanese Patent Application Laid-Open No. 8-35860).
To make the thermistor unaffected by external noises on the signal line of the thermistor, measures may be taken, such as taking a distance from other signal lines which are noise sources or buffering a signal from the thermistor by an amplifier to transmit it.
However, since a material which is difficult to oxidize such as gold is expensive, a connector using such a material is greatly disadvantageous in cost. In the case of the connector having the structure in which the oxide film is destroyed by damaging the surface of the connector during fitting, if the connector is repeatedly attached/detached, the contact point to be conductive is damaged. Thus, the connector having such structure may not be suitable for, for example, a portion to be repeatedly attached/detached.
A circuit that supplies a current to destroy the oxide film has been put into practical use. However, the circuit has a problem in that when a temperature detection device represented by a thermistor is connected, increase in an amount of current leads to self-heating of the temperature detection device, and accuracy of temperature detection is lost.
For example, in the case of the thermistor installed within the fixing device of the image forming apparatus, when a detected temperature rises by an amount of self-heating by the thermistor, the fixing device is controlled at a temperature lower than the original target temperature, and it may cause deterioration in image quality such as an occurrence of a fixing failure of a toner image on the transfer material.
Taking a distance from the other signal lines which are noise sources to make the thermistor unaffected by external noises on the signal line of the thermistor hinders miniaturization of the apparatus.
Buffering the signal of the thermistor by the amplifier to transmit it will increase a cost of the circuit and space for an electric component.